This invention relates to probe positioners and more particularly to remotely controlled probe positioners for inspecting nuclear steam generators.
There are many situations in which a hazardous environment limits human access to various locations. One such situation occurs in the inspection and repair of nuclear steam generators. A typical nuclear steam generator comprises a vertically oriented shell, a plurality of U-shaped tubes disposed in the shell so as to form a tube bundle, a tube sheet for supporting the tubes at the ends opposite the U-like curvature, and a dividing plate that cooperates with the tube sheet forming a primary fluid inlet plenum at the one end of the tube bundle and a primary fluid outlet plenum at the other end of the tube bundle. The primary fluid having been heated by circulation through the nuclear reactor core enters the steam generator through the primary fluid inlet plenum. From the primary fluid inlet plenum, the primary fluid flows upwardly through first openings in the U-tubes near the tube sheet which supports the tubes, through the U-tube curvature, downwardly through second openings in the U-tubes near the tube sheet, and into the primary fluid outlet plenum. At the same time, a secondary fluid, known as feedwater, is circulated around the U-tubes in heat transfer relationship therewith thereby transferring heat from the primary fluid in the tubes to the secondary fluid surrounding the tubes causing a portion of the secondary fluid to be converted to steam. Since the primary fluid contains radioactive particles and is isolated from the secondary fluid by the U-tube walls and the tube sheet, it is important that the U-tubes and a tube sheet be maintained defect-free so that no breaks will occur in the U-tubes or in the welds between the U-tubes and the tube sheet thus preventing contamination of the secondary fluid by the primary fluid.
Occasionally, it is necessary to either inspect or repair the U-tubes or tube sheet welds by way of access through the primary fluid inlet and outlet plenum. For this purpose, manways are provided in the vertical shell so that working personnel may enter the inlet and outlet plenum to perform operations on the U-tubes and the tube sheet. However, since the primary fluid which is generally water contains radioactive particles, the inlet and outlet plenum become radioactive which thereby limits the time that working personnel may be present therein. Accordingly, it would be advantageous to be able to perform operations on the U-tubes and the tubesheet without requiring the presence of working personnel. There are several mechanisms known in the art that attempt to provide a solution to this problem, but none of them have been able to completely solve the problem.
What is needed is a remotely operable probe positioner that is capable of positioning a probe for inspecting the tube sheet or U-tubes in a nuclear steam generator without subjecting working personnel to great radiation hazards.